Over the past thirty years, significant attention has been given to the production of ethyl alcohol, or “ethanol,” for use as an alternative fuel. Ethanol not only burns cleaner than fossil fuels but also can be produced using grains such as corn, which is a renewable resource. Further, the production of ethanol results in new sales outlets for corn, provides additional jobs, and reduces the nation's dependency on foreign oil.
Ethanol is typically produced from corn through either a wet or dry milling process. In the wet milling process, the corn kernel is separated into various components including germ, starch, protein, and fiber, resulting in several co-products. For example, separated germ may be further processed for oil recovery; starch may be saccharified and fermented for ethanol production; and protein and fiber may be used as feed material. In a dry mill process, whole corn is ground, treated with enzymes, and cooked. The resulting “mash” is treated with enzymes to further break down the starchy endosperm tissue into glucose. The converted mash is fermented and distilled, producing ethanol, carbon dioxide, and distiller's dried grains (“DDG”), which are the undissolved solid components (i.e. stillage) remaining in the fermentation tank after the broth is removed. Thus, DDGs are typically comprised of yeast and unfermented components of the corn.
The Dry grind process converts corn into two products, including ethanol and distiller's grains with solubles. If sold as wet animal feed, the co-product is known as distiller's wet grains with solubles (“DWGS”). Conversely, if dried for animal feed, the co-product is known as distiller's dried grains with solubles (“DDGS”). In the standard dry grind ethanol process, one bushel of corn yields approximately 8.2 kilograms (i.e. approximately 18 pounds) of DDGS in addition to the approximately 10.2 liters (i.e. approximately 2.7 gallons) of ethanol. These co-products provide a critical secondary revenue stream that offsets a portion of the overall ethanol production cost.
Typical ethanol plants employ scrubbers on the vent systems coupled to the fermentation vessels and other process gas discharges to minimize the discharge of vaporous ethanol and other volatile organic compounds (“VOCs”) from escaping into the atmosphere with the carbon dioxide and other vapors. As appreciated by those skilled in the art, VOCs may include compounds such as acetaldehyde, etheyl acetate, acrolein, and acetone. These scrubbers use fresh water along with a variety of additives to increase the solubility of the ethanol and VOCs. The precipitate of the scrubber is water with a low concentration of ethanol. This water mixture is then reintroduced into the process as make-up water which is then taken through the heating cycles along with the mash. It is in this area that the temperature is often taken over the flash point of ethanol, which in turn allows the ethanol to be lost to a vent system and destroyed in a thermal oxidizer. Consequently, both ethanol and VOCs are discharged into the atmosphere which reduces the ethanol yield and pollutes the environment.
Thus, what is needed is an improved system and method for extracting ethanol and VOCs from a vent stream during ethanol production.